1. Field of the Invention
The present invention generally relates to dc-dc converters and, more particularly, to a three-level dc-dc converter which uses an auxiliary voltage source and flying capacitor to reduce the voltage stress across the power switches, improves the efficiency by reducing the circulating energy in the converter, and achieves zero voltage, zero current switching (ZVZCS).
2. Description of the Prior Art
DC-DC power converters are required for many modern day applications. For example, such converters are required for high voltage, high power applications such as used in telecommunications, battery chargers, and uninterruptible power supplies with power factor correction.
The Power Factor Correction (PFC) stage is important stage for the future generation of three phase distributed power systems due to the IEC61000-3-2 Class A standard. The three-phase boost rectifier is an interesting option for this PFC stage, since it can easily comply with the aforementioned standard with simplicity, efficiency, reliability, and low cost. However, in order to reduce the harmonic distortion in this converter, its input to output voltage characteristic has to be increased. Therefore, the dc/dc step down second stage converter usually has to support voltage stress up to 800 Volts, and the need for high voltage devices is required. Unfortunately, these high voltage devices present a poor behavior in terms of conduction and switching losses, and also have a higher cost.
The so-called Zero Voltage Switching (ZVS) Three-Level (TL) dc/dc converters and the Dual Bridge dc/dc converter have overcome this problem. These dc/dc converters reduce the voltage stress across the power switch to half of the input voltage. Hence, power devices with better characteristics can be used. Also, these converters achieve zero voltage switching for the primary switches, which is a necessary characteristic in order to increase the efficiency in the converter.
Examples of three-level ZVS-PWM converters are presented by J. Renes Pinheiro et al., Three Level ZVS PWM Converter A New Concept in DC-to-DC Conversion, IEEE IECON Record, 1992, and J. Renes Pinheiro et al., Wide Load Range Three-Level ZVS-PWM DC-to-DC Converter, IEEE PESC Record, 1993.
FIG. 1 shows a prior art three level ZVS-PWM converter as discussed above. It comprises a main computation leg formed by four MOSFET switches (M1, M2, M3, and M4), four diodes (D1, D2, D3, and D4), and four capacitors (C1, C2, C3, and C4). The diodes D1-D4 and the capacitors C1-C4 are intrinsic elements of the MOSFET switches M1-M4. Lr is a commutation inductor which is comprised of an external inductor plus the leakage inductor of the isolation transformer Tr. The capacitors Ca1 and Ca2 and the inductor La form a commutation auxiliary circuit. The output stage is formed by rectifiers Dr1, and Dr2 and by an output filter formed by Lr and Cf. Ro is the load and Dc1, and Dc2, are clamping diodes.
In order to insure the commutation ZVS at a wide load range , it is necessary to have enough energy to charge and discharge the intrinsic capacitances C1-C4. The switches of the converter operate under different conditions of commutation. The commutation of the outer switches M1 and M4 always takes place when the current through the primary transformer is equal to the output current reflected to the primary. Since the filter inductance current reflected to the primary is large, even for small primary current, such as the transformer magnetizing current, it is sufficient to perform the commutation. On the other hand, the inner switches M1 and M3 commutate when the transformer is short-circuited by the output rectifier. Thus, it is necessary to use an auxiliary inductor La to perform the commutation, since the resonant inductor Lr alone does not provide enough stored energy.
As can be seen from above, one of the drawbacks that these prior art converters present is the use of the energy stored in the transformer leakage inductance to achieve zero voltage switching for two of the switches. Hence, an additional resonant inductance has to be included in order to assure ZVS for almost the entire load range. This in turn increases the circulating energy, which increases the conduction losses in the converter. In addition, both a reduction in the effective duty cycle and a severe secondary parasitic ringing occur.
It is therefore an object of the present invention to provide a Three-level (TL) DC-DC converter that overcomes the drawbacks presented by the Zero Voltage Switching (ZVS). Three-level converter, such as high circulating energy, severe parasitic ringing on the rectifier diodes, and limited ZVS load range for the inner switches.
According to the invention, a three-level DC-to-DC converter is provided having zero-voltage and zero-current switching (ZVZCS). A primary (input) stage and a secondary (output) stage are separated by an insolation transformer. Four commutation switches on the primary side are connected in cascade fashion and are switched by phase-shift control. An input dc voltage source is connected between the first and fourth switches. Two capacitors are also connected between the first and fourth commutation switches and function to split the input voltage in half. The primary side of the isolation transformer is connected at one end between the two capacitors and at its second end between the second and the third commutation switches. Two clamping diodes are also provided; one between the first and second switches and the two capacitors and the other between the third and fourth switches and the and the two capacitors. A flying capacitor is connected at one end between the first and second commutator switches and at a second end between the third and fourth commutator switches.
The output stage comprises two rectifiers connected to either end tap of the secondary side of the isolation transformer and an output filter comprising an a resistor and capacitor connected at one end to the rectifiers and at a second end to the center tap of the isolation transformer. An auxiliary voltage source is additionally added between the rectifiers and the center tap of the isolation transformer.
In operation, the positive and negative charges on the plates of the flying capacitor constantly switch sides and the as the first and second, and third and fourth commutation switches switch on and off. In this manner the primary side achieves Zero Voltage Switching (ZVS) for the outer commutation switches. The flying capacitor is an important element to achieve ZVS for the switches. This flying capacitor provides a trajectory for the current in order to charge and discharge the parasitic capacitance of the switches S1 and S2, respectively. Also, this flying, capacitor allows operation of the converter with a phase-shift control. Without this flying capacitor, the zero voltage operation would not be achieved since the outer switches would be turned on with xc2xc of the input voltage.
In addition, during freewheeling (i.e., when no power is being transferred from the primary side to the secondary side), the auxiliary power source is switched on to provide a voltage which is reflected back to the primary side in order to eliminate the circulating energy and also to achieve Zero Current Switching (ZCS) for the commutation switches. Of course, the proposed converter would operate just with ZVS if the auxiliary voltage source is not provided.